1. Field of the Invention
This invention relates to an optical pickup direction detection unit and an optical pickup direction detection method and more particularly to, in an optical pickup for wiring information onto an optical disc based on emission light from a light source or reading information from an optical disc based on return light from the optical disc, an optical pickup direction detection unit and an optical pickup direction detection method for determining the direction to move the optical pickup in order to position emission light on an information record track of the optical disc.
2. Description of the Related Art
An optical pickup comprising a light source, an optical system, and a light detection system is used to apply emission light from the light source to the information record side of the surface of an optical disc such as a CD (compact disc) or a CD-ROM (compact disc-read-only memory) for writing optical disc record information of music, data, etc., or read optical disc record information from light reflected on and returned from the information record side of an optical disc.
To reliably write the information onto the optical disc or reliably read the information from the optical disc in the optical pickup, it is necessary to control so as to always apply emission light onto a record location on the information record side of the optical disc, such as an information record track, namely, it is necessary to perform tracking servo control, and it is necessary to control so as to converge emission line on a record location as a spot-like point, namely, it is necessary to perform focusing servo control.
A tracking servo control technique, for example, as shown in FIGS. 11(A) to 11(D) is known.
FIG. 11(D) is a sectional view of an optical disc wherein the surface of the optical disc is cut in the radial direction of the disc. A projection portion L is called a land and a recess portion G is called a groove. Information is written into and read from the grove. If the optical pickup is moved in the radial direction of the disc (right and left in FIG. 11), emission light from the optical pickup moves so as to cross the land L and the groove G.
At this time, a reproduction (playback) RF signal from a photodetector receiving return light becomes as shown in FIG. 11(A). If the reproduction (playback) RF signal is made to pass through a low-pass filter (LPF) or the envelope of the signal is taken, a radial contrast signal RC as shown in FIG. 11(B) is provided. As shown here, the radial contrast signal RC has the center of the crest of the waveform matching the center position of the land L and the center of the trough of the waveform matching the center position of the groove G.
Light from the photodetector receiving return light is converted into an electric signal, whereby a tracking error signal TE as shown in FIG. 11(C) can be provided. The tracking error signal TE has a waveform passing through a zero level B (crossing zero) at the point corresponding to the center position of the groove G as shown in the part of A.
Therefore, to perform tracking servo control, first the optical pickup is moved in any direction, position in the radial direction based on the slope of the tracking error signal TE (differential coefficient) so as to become the center of a predetermined trough of the radial contrast signal RC. This operation is called xe2x80x9cbringing into tracking servo controlxe2x80x9d. Then, position control is performed so that the tracking error signal TE crosses zero, whereby tracking servo control is performed.
However, as optical discs with information recorded at a high density, such as a DVD (digital versatile disc), have been used in recent years, the following problem arises in the tracking servo control of the optical pickup in the related art described above:
As information is recorded on a disc at a higher density, the track pitch of land L and groove G (the distance between the center positions in FIG. 11(D)) becomes narrower and a limitation is imposed on the groove depth, etc. Thus, the asperities of the waveform of the radial contrast signal RC become very small, the S/N ratio of the signal lowers, and the position of each trough of the radial contrast signal RC may be detected erroneously. If the position of the trough of the radial contrast signal RC is detected erroneously, the direction to move the optical pickup is determined erroneously and it becomes hard to bring into tracking servo control at a predetermined position; tracking control becomes unstable.
It is therefore an object of the invention to provide an optical pickup direction detection unit and an optical pickup direction detection method for making it possible to bring into tracking servo control stably if the track pitch of an optical disc is narrow.
In order to achieve the above object, according to the invention, there is provided, in an optical pickup for writing optical disc record information onto an information record track of an optical disc having prepits based on emission light from a light source or reading the optical disc record information from return light provided by reflecting emission light from the light source on the information record track, an optical pickup direction detection unit for detecting a direction to move the optical pickup in order to position the emission light on the information record track, the optical pickup direction detection unit comprising: a photodetector for receiving the return light and outputting a light detection electric signal; push-pull signal generation means for generating a push-pull signal from the light detection electric signal from the photodetector; prepit signal generation means for extracting a prepit component from the push-pull signal and generating a prepit signal; tracking error signal generation means for generating a tracking error signal from the light detection electric signal from the photodetector; and optical pickup move direction detection means for detecting the move direction of the optical pickup based on the tracking error signal and the prepit signal.
Further, according to the invention, there is provided, in an optical pickup for writing optical disc record information onto an information record track of an optical disc having prepits based on emission light from a light source or reading the optical disc record information from return light provided by reflecting emission light from the light source on the information record track, an optical pickup direction detection method for determining a direction to move the optical pickup in order to position the emission light on the information record track, the optical pickup direction detection method comprising the steps of: receiving the return light and outputting a light detection electric signal; generating a push-pull signal from the light detection electric signal; extracting a prepit component from the push-pull signal and generating a prepit signal; generating a tracking error signal from the light detection electric signal; and detecting the move direction of the optical pickup based on the tracking error signal and the prepit signal.